It is increasingly common to interconnect several fixed and mobile electronic appliances, such as computers and peripherals therefor, handheld digital devices like personal digital assistants and cell phones, data collection devices and sensors for example for area videosurveillance, and so on. Said devices comprise the nodes or stations or units or terminals of the communication system. Term network will sometimes be used herein as a synonymous of term communication system.
Such networks can be of various geographical size, ranging from BAN (Body Area Network), PAN (Personal Area Network), LAN (Local Area Network), MAN (Metropolitan Area Network), to WAN (Wide Area Network).
The connection or links among the nodes may exploit several physical media, such as electric wires, buses and optical fiber, as well as wireless media such as radio frequency and infrared electromagnetic radiation.
While wired communication systems are still broadly used, wireless and hybrid type communication systems are increasingly common due to their flexibility of installation, allowing for mobile devices to interconnect with each other and/or with an infrastructure.
Besides the physical medium, to effectively allow exchange of information, the nodes of a communication system should share a network protocol, including e.g. the formats of data packets and node addresses. Where the transmission medium is shared, a Medium Access Control (MAC) protocol is also needed to allow for all nodes to successfully communicate within the system, avoiding colliding communications to the greatest possible extent.
Several Medium Access Control (MAC) protocols are well known in the art.
In centralized MAC protocols, a central node coordinates all the communication among the other nodes—and possibly itself—in the communication system. On the other hand, distributed MAC protocols provide for distributed coordination among the nodes themselves. Hybrid MAC protocols are also known.
For example, the common and widely deployed Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard for wireless networks, commercially known as Wi-Fi (Wireless Fidelity), provides a distributed MAC protocol, namely Distributed Coordination Function (DCF), where the access to the physical medium is arbitrated by a random contention among the wireless nodes. Briefly stated, each node only transmits when it senses the medium free. Such MAC protocol suffers from the well-known hidden node and exposed node problems.
With reference to FIG. 1, consider a communication system having nodes 10, 12, 14, 16 and respective transmission ranges 18, 20, 22, 24. The transmission ranges partly overlap so that there is a link 26 between nodes 10 and 12, a link 28 between nodes 12 and 14, and a link 30 between nodes 14 and 16, but no links between nodes 10 and 14. Both nodes 10 and 14, when wishing to communicate towards node 12, being hidden to each other, will sense the medium free and start communication. A communication collision will however occur at node 12. To alleviate such problem, the DCF protocol of the IEEE 802.11 standard itself incorporates an optional mechanism based on the exchange of RTS/CTS (Request To Send/Clear To Send) messages for reserving the communication medium. According to this embodiment of the DCF protocol, node 10 transmits a short RTS message to node 12, and node 12 transmits in reply a short CTS message, both messages indicating the duration of the desired communication from node 10 to node 12. Node 14, receiving the CTS message, will refrain from communicating to node 12 for such duration.
Again with reference to FIG. 1, the exposed node problem arises when node 12 is transmitting to node 10. Node 14 will sense the medium busy and refrain from transmitting to node 16, even if no collision would occur at either nodes 10 and 16. The communication system's performance is therefore deteriorated.
The IEEE 802.11 standard further provides for a MAC protocol, namely Point Coordination Function (PCF), according to which a “master” node grants centralized scheduled access to “slave” nodes in a poll list during a period (CFP) alternating with a period (CP) in which the rules of the DCF protocol are used, and nodes may request to be added to the poll list. Thus, IEEE 802.11 PCF standard is a partly centralized and partly distributed MAC protocol, i.e. a hybrid MAC protocol as term “hybrid” is used herein. The Applicant notes that PCF is optional, and only very few Access Points (APs) or Wi-Fi adapters actually implement it.
An example of a centralized MAC protocol is the IEEE 802.16 standard, commercially known as WiMAX, where all the communications occur only from the Base Station (BS) acting as master to the Subscriber Stations (SSs) acting as slaves and viceversa in a polling fashion. This approach naturally prevents hidden node or exposed node problems from occurring. However, no direct communication is possible between any two SSs, even if they are within the transmission range of each other and could therefore hear each other directly. This constraint may negatively impact on the system performance and efficiency.
Another example of a centralized MAC is the Bluetooth protocol, wherein a node may become either a slave or the master of up to eight slave nodes, granting them access to the medium in a polling fashion.
While in some communication systems or networks the choice of which node is to act as the master of a centralized MAC protocol is dictated by the network structure, purpose and/or nature, in ad hoc networks of peer nodes any node may in principle be elected as the master. Several algorithms and approaches for master election have been proposed in a wide variety of contexts.
Besides the IEEE 802.11 PCF protocol, other hybrid MAC protocols have been proposed to improve performance of communication systems.
Furthermore, there is often the need to easily establish a communication system among mobile nodes, or nodes to be fixedly deployed without the knowledge a priori of the connectivity graph or physical topology.
The Applicant faced the problem of providing for automatic selection of a MAC protocol that increases a communication system's performance.